NODE-2600 Optionally override a blockchain state in /utils/script/evaluate

[vsuharnikov]

Now a user can specify a portion of blockchain state to override in a body of REST API request: /utils/script/evaluate.
Only asset balances is supported for now.

{
  "call": { /*...*/ }, // or "expr"
  // ...,
  "state": {
    "accounts": {
      "$address1": {
        "regularBalance": 11111, // An optional field
        "assetBalances": { // An optional field
          "$assetId1" -> 22222,
          "$assetId2" -> "33333" // Strings as values are supported as well
          // ...
        }
      },
      "$address2": {
        // "regularBalance": 11111,
        "assetBalances": {
          "$assetId2" -> "33333"
          // ...
        }
      },
    }
  }
}